GO-6869 Exclude nightly tags from git describe in CI workflows

[requilence]

Summary

• Add --exclude='*-nightly.*' to git describe in build.yml, perftests-grafana.yml, and perftests.yml
• Fixes non-deterministic tag selection when a nightly tag and a release tag coexist on the same commit
• nightly.yml already computes its version explicitly and is unaffected

Problem

git describe --tags --always picks tags non-deterministically when multiple tags point to the same commit. A nightly tag like v0.44.0-nightly.20260307.1 can be selected over the actual release tag v0.49.0, causing the built binary to report an incorrect version.

Fix

# Before
GIT_SUMMARY=$(git describe --tags --always)

# After
GIT_SUMMARY=$(git describe --tags --exclude='*-nightly.*' --always)

Test plan

• Trigger a release build on a commit that has both a nightly tag and a release tag — verify the binary reports the release version

[github-actions]

New Coverage	46.6% of statements
Patch Coverage	0.0% of changed statements (0/0)

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